Hybrid integrated circuits used in automotive applications often employ surface-mounted integrated circuit packages. Such packages are typically electrically interconnected with their respective conductors on the substrate of the hybrid integrated circuit using wire bonding techniques, in which a very thin electrically-conductive wire is bonded to the lead of the package and to a bond pad soldered to the conductor. Bond pads serve to promote a durable and reliable bond with the wire, which is typically an aluminum alloy, and with the conductor. The conductor is typically formed from a different material than that of the wire. Common conductor materials are silver and silver alloys such as a silver-palladium alloy, though the use of other metals such as copper or a copper alloy is also known for this purpose.
In order to be compatible with the two different materials used for the wire and conductor, the two bonding surfaces of conventional bond pads are typically composed of different materials, each of which is compatible with either the wire material or the conductor material. Because the wire material is typically an aluminum alloy and the conductor material is typically silver or a silver alloy, prior art bond pads are typically composed of a multi-layer structure wherein an intermediate steel substrate is layered with aluminum or an aluminum alloy on one side to bond to the wire bond, and layered with copper or a copper alloy on the opposite side to bond with the silver or silver alloy conductor. Such bond pads may be on the order of about 0.25 millimeter thick, with the steel substrate accounting for about 0.15 to about 0.18 millimeter of that thickness. The thickness of the aluminum alloy layer may be on the order of about 25 to about 75 micrometers, while a thinner layer of copper is typically used, on the order of about 25 to about 50 micrometers. Bond pads of this type are placed with the copper layer face down on the conductor, in order to allow soldering of the copper layer to the silver or silver alloy conductor. Consequently, the bond pad is oriented such that the aluminum alloy layer is face up, in order to allow bonding of the aluminum alloy layer to the aluminum wire.
Due to the small size of integrated circuit packages and the numerous leads typically required for such packages, the maximum allowable size for a bond pad is often severely restricted. As an example, bond pads are typically on the order of about two millimeters to a side, though the size may vary depending on the application. The extremely small size of bond pads complicates their handling and placement during assembly, which typically is done by vacuum-operated pick-and-place devices. In particular, it has been found that bond pads are occasionally placed upside down on the conductor pattern. When this occurs, the aluminum alloy layer is mated with the silver or silver alloy conductor, and the copper alloy layer is mated with the aluminum wire, such that reliable and durable bonds are unlikely to be produced due to the material mismatch of the bond surfaces during soldering or bonding. As a result, the integrity of the bonds suffer.
Thus, it would be desirable if the occurrence of upside down bond pads could be substantially reduced if not eliminated during the assembly of a hybrid integrated circuit. A suitable solution would not significantly complicate the manufacturing or handling of the bond pad, nor have a detrimental effect on the ability of the bond pad to form high integrity bonds with the wire and conductor. Furthermore, the solution must be highly repeatable and reliable when used in mass production, particularly for the manufacture of electronic hardware for the automotive industry.